This invention relates to centrifugal turbomachines, and, more specifically, to diffuser structure for use in such devices.
In centrifugal turbomachines such as gas compressors, the kinetic energy of the flowing medium which is issuing at high speed from the impeller is converted into pressure energy and the efficiency and stability of the compressor is dependent upon the means for converting the kinetic energy into static pressure. One of the major problems arising in the use of centrifugal gas compressors for applications where the compression load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller, and diffuser passage must be sized to provide for the maximum volumetric flow rate desired. In centrifugal refrigerant compressors, the loads typically vary over a wide range and they may be operated at such low flow rates that their diffusers are too large for efficient operation. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, flow in both the impeller and diffuser becomes separated from the wall along the entire length of the flow passage and there appears to be a partial reversal of flow in the diffuser passage creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the machine.
Many high-performance centrifugal stages employ a fixed vane diffuser section to achieve the kinetic energy conversion since a vaned diffuser is more efficient at designed incidence than a vaneless diffuser. The low flow limit corresponds to the onset of a surge or stall condition which occurs as the fluid flow from the impeller becomes more tangential as the flow decreases. This produces a large flow angle and magnitude with respect to the leading edge of the fixed diffuser vanes, creating a violet instability. The high flow limit corresponds to a choke condition caused as increasing fluid flow from the impeller becomes more radial and finally chokes the diffuser throat with very large kinetic energy loss. Since a vaneless diffuser has better off-design performance than a vaned diffuser, because it does not suffer from incidence losses, it is often chosen where there is considerable off-design operation.
Various techniques have been used to increase the range between the surge and choke limits of a compressor. Guide vanes in the inlet of the compressor have been employed to vary the flow direction and quantity of entering gas. Movable diffuser vanes have also been employed to permit alignment of the vanes with the changing flow direction as the flow rate changes.